Modern satellite communication systems provide a pervasive and reliable infrastructure to distribute voice, data, and video signals for global exchange and broadcast of information. These satellite communication systems have emerged as a viable option to terrestrial communication systems. Satellite communication systems are susceptible to service disruptions stemming from changing channel conditions, such as fading because of weather disturbances. Additionally, such systems cannot readily increase capacity as the number of satellite transponders is fixed. Further, channel interference constrains system capacity. As a result, efficient frequency reuse schemes are vital to the profitability of these satellite communication systems.
FIG. 8 is a diagram of a conventional satellite system in which inbound and outbound signals utilize unique frequency assignments. A two-way satellite system 800 includes a hub station 801 that transmits outbound signals to a satellite 803 over a first carrier frequency, f1, and receives inbound signals from the satellite 803 over a second carrier frequency, f2. Concurrently, the satellite 803 can communicate with a remote satellite terminal 805, which utilizes two other frequencies, f3, and f4, to transmit and receive, respectively. This arrangement is typical of a two-way satellite communication system, whereby the hub station 801 transmits content to multiple Very Small Aperture Terminals (VSATs) 805 (in which one is shown). The use of unique frequencies by the terminal 805 and the hub station 801 ensures that channel interference is minimized. The drawback, however, is that a large number of frequencies are required when terminals are added to the system 800. As spectrum is a precious resource, it is vital to use the spectrum efficiently.
An improvement to the system 800 requires sharing of the satellite transponder for the inbound signals and the outbound signals. The efficiency of the spectrum sharing can be measured in the total throughput achieved by the inroute and outroute. Alternatively, if the outbound throughput is maintained at the same level as that of system without sharing the spectrum with the inroutes, the throughput achieved by the inbounds are gained by the system. Different schemes will yield different gains. In particular, when compared with traditional systems, significant gain can be realized by properly modeling and compensating the impact of the transmission channel. Conventional approaches assume that both inbounds and outbound share an ideal linear channel. As a result of this assumption, large uncompensated mutual interference exists between the inbound signals and the outbound signals.
Conventionally, to mitigate this mutual interference, spread spectrum techniques are utilized, wherein the average energy of the inbound signal is spread over a bandwidth that is much wider than the information bandwidth. Using spread spectrum transmission in the same transponder for both the inbound and outbound signals conserves space segment resources. However, transmitted power levels must be very low in order to minimize interference to the forward link; and as a result, spread spectrum techniques results in very limited capacity of each link, such that information bit rates on the return links tend to be low.
Furthermore, spread spectrum inbound signals are deployed to combat the channel impairments. A drawback with this approach is that overall system capacity is reduced. In addition, the impairments are greater if the inbound signals are Time Division Multiple Access (TDMA)-based instead of Code Division Multiple Access (CDMA)-based. In particular, it is recognized that the communication channels within the system 800 may exhibit non-linear characteristics, notably from the amplifiers within the transponders. Conventional systems fail to compensate for this non-linear behavior. Further, the transponder introduces group delay stemming from a noise-limiting filter applied before the amplifier. The non-linear effects and the group delay impede performance of a shared transponder scheme. It is noted that, in general, a number of techniques exist for compensating non-linear effects of an amplifier. However, conventional techniques are not applicable to spectrum sharing. In the spectrum sharing situation, the impact of these channel impairment exhibits completely different behaviors. Such channel impairment needs to be compensated before the interference suppression techniques can be applied.
Based on the foregoing, there is a need for a radio communication system that enhances system capacity. There is also a need to minimize the effects of non-linearity of the communications channel.